This invention relates to a coulometric titration method, more particularly a coulometric titration method capable of measuring a minute quantity of substance at high accuracies.
An electric titration method has been well known comprising the steps of introducing into a titration cell a composition to be titrated in a sample or after converting the composition into a titratable compound, together with a carrier gas, passing electric current between opposing generating electrodes disposed in the titration cell so as to eliminate a potential variation of an electrolyte contained in the cell caused by the introduction of the composition to be titrated, integrating the electric current and calculating the quantity of the composition to be titrated from the integrated value according to the law of Faraday. With this method, however, it is necessary to bring the apparatus to a stable state by passing the carrier gas through the apparatus prior to the measuring. Even in this step, since the potential of the electrolyte varies slightly, it is necessary to maintain the potential of the electrolyte at a preset potential, i.e., an end potential by passing across the electrodes a current proportional to the difference between the potential of the electrolyte detected by a detecting electrode and the end potential.
As the apparatus becomes stable, the current flowing through the generating electrodes becomes a constant value, termed a blank current.
The sample is injected into or admixed with the carrier gas and the resulting mixture is introduced into the electrolyte directly or after converting the sample into an electrolizable compound. Then, as the potential of the detecting electrode varies, a titration current proportional to the difference between the detected potential and the end potential is passed across the generating electrodes so as to restore the detected potential to the end potential. A condition in which the detected potential reaches a value before sample introduction is taken as the completion of the titration and the difference between the titration current passed across the generating electrodes during the titration and the blank current is integrated to calculate the quantity of the composition to be titrated.
When the sample is introduced at a constant rate, the composition to be titrated is introduced into the electrolyte thus increasing the potential thereof. However, since the composition to be titrated in the electrolyte is immediately consumed as a result of the titration, the potential of the electrolyte would be maintained at a value slightly higher than the end point potential while the component to be titrated is being introduced. Accordingly, the current is also maintained at a value slightly higher than the blank current in proportion to the variation in the potential. Upon completion of the introduction of the sample, the quantity of the component introduced decreases so that the current also decreases back to the blank current.
Although this method is used widely, when the injected quantity of the composition to be titrated is small, the blank current becomes large relative to the titration current thus degrading the accuracy of measurement.